<!DOCTYPE html>
<html>
	<head>
		<meta charset="utf-8">
		<title></title>
		<script type="text/javascript">
			function f0(){
				alert("这是额外执行的函数")
			}
			function f1(){
				alert("按钮1被点击了")
			}
			function f2(){
				alert("按钮2被点击了")
			}
			function f3(){
				alert("按钮3被点击了")
			}
			function f4(){
				alert("按钮3不能被点击了")
				let b3 = document.getElementsByClassName("b3")[0]
				b3.removeEventListener("click",f3)
			}
		</script>
	</head>
	<body onclick="f1()">
		<button onclick="f1(),f0()">按钮1</button>
		<button id="b2">按钮2</button>
		<button class="b3">按钮3</button>
		<button id="b4">按钮4</button>
		<script type="text/javascript">
			let b2 = document.getElementById("b2")
			b2.onclick = f2
			
			let b3 = document.getElementsByClassName("b3")[0]
			b3.addEventListener("click",f3)
			b3.addEventListener("click",f0)
			
			let b4 = document.getElementById("b4")
			b4.onclick = f4
			
		</script>
	</body>
</html>
